question about semaphores

Code:

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typedef struct {
    int buf[BUFF_SIZE];  /* shared var */
    int in;                  /* buf[in%BUFF_SIZE] is the first empty slot */
    int out;                  /* buf[out%BUFF_SIZE] is the first full slot */
    sem_t full;              /* keep track of the number of full spots */
    sem_t empty;              /* keep track of the number of empty spots */
    sem_t mutex;              /* enforce mutual exclusion to shared data */
} sbuf_t;

sbuf_t shared;



int main()
{
    pthread_t idP, idC;
    int index;

  sem_init(&shared.full, 0, 0);
    sem_init(&shared.empty, 0, BUFF_SIZE);

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At the red lines,I dont understand why the value of shared.full is 0 and the value of shared.empty is BUFF_SIZE. I need some explanations. Thanks

Well sem_init full is a counter. I believe your comment for this was enough, so I am not sure where your confusion may be with that. This is just the initial value being used. As far as sem_init() making use of the constant BUFF_SIZE, this may be due to the fact that the function call is using this value argument and subtracting from this.


Have you read up on this call?
sem_init

Quote:

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Originally Posted by slingerland3g

Well sem_init full is a counter. I believe your comment for this was enough, so I am not sure where your confusion may be with that. This is just the initial value being used. As far as sem_init() making use of the constant BUFF_SIZE, this may be due to the fact that the function call is using this value argument and subtracting from this.


Have you read up on this call?
sem_init

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but why dont we do another way around like

Code:

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sem_init(&shared.full, 0, BUFF_SIZE);
sem_init(&shared.empty, 0, 0);

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I think there is another reason for that but I can not get it...